The present invention relates to a heat exchanger having a network of heat exchanging elements which includes a plurality of pipes of oval cross-section and end plates provided with a plurality of openings for receiving end portions of the oval pipes, a plurality of sealing elements arranged in the openings and the end plates being attached to the end portions by expanding the end portions of the oval pipes. The invention also relates to a method for liquid-tight mounting of at least one end plate to the network of heat exchanging elements.
The mounting of heat exchanging networks on end plates of a heat exchanger, such as used for example in a motor vehicle radiator still involves problems when the pipes of the heat exchanging network have an oval cross-section and are supposed to be secured to the end plates by expanding their end portions only, that means without the use of any soldering, cementing and the like. Inasmuch the end plates serve for supporting a collecting vessel which accumulates the cooling medium flowing through the pipes of the heat exchanger, the mounting of the end plate must be not only liquid-tight or gas-tight but also mechanically stable to such an extent that the unavoidable shaking or vibrations resulting during the use of the heat exchanger do not impair the mount and can withstand the hydraulic pressure of the cooling medium. This condition, particularly when using oval pipes as heat exchanging elements cannot be always met because the ratio of diameters of the oval pipes that means the ratio of the maximum outer diameter to the minimum outer diameter is greater than about 2.5:1. In heat exchanging pipes of this kind, the fastening of long pipe wall portions extending substantially parallel or in the direction of the maximum diameter, in the elastic sealing collars of the end wall, was frequently insufficiently strong. In spite of the expansion of the end portions of the pipe there is still the risk that the pipe wall portions extending parallel or in the direction of the miminum pipe diameter collapse and thus preclude a reliable seal or a mechanically stable connection.
The prior art heat exchanger of this kind (DE-OS 27 47 275), therefore have reinforced long sides of the heat exchanging pipes, by providing for example wall portions of greater thickness or annular inserts or a step-wise enlargement. Such conventional measures however have proved as unsatisfactory for a mass production.
For avoiding this disadvantage a heat exchanger has been proposed (DE-PS 1,751,710) wherein the bottom end plate is provided with circular cutouts or openings for receiving sealing elements also of a circular cross-section and the throughgoing oval end portions of the heat exchanging pipes have been expanded to form a cylindrical or cone configuration. By this measure the risk of collapsing is practically eliminated. Of course, it must be taken into account that such prior art heat exchanger and the corresponding method for mounting the heat exchanging pipes is suitable only for oval pipes having the diameter ratio approximately 2:1. When using the ratio of maximum to the minimum diameter of 2.5:1 and more, as required for heavy duty motor vehicle radiators and the like, such expansion of end portions of the pipes results in excessively long transition zones between the pipe portion having a circular cross-section and the pipe portion having an oval cross-section. This brings about the danger that the pipes themselves or the sheet metal fins drawn thereon crack or rupture or the immediately adjoining sheet metal fin is shifted and thus the cooling efficiency is decreased. In addition the use of cylindrical end portions of the heat exchanging pipes in a series of side-by-side arranged heat exchanging networks necessitate substantially increased spacing between the pipes in comparison with oval configuration of the end portions and consequently a corresponding reduction of efficiency of heat exchanging network in a heat exchanger of the same size would result. Such substantial disadvantages can be only insignificantly reduced by using heat exchanging pipes of an increased maximum diameter corresponding to the diameter of the sealing collar, and shaping the end portions of the pipe first by means of an inserted expansion mandrel and the like in such a manner that the maximum diameter is reduced and the minimum diameter is increased to provide the cylindrical cross-sections in the end portions which are slightly smaller in diameter than the sealing collars. Even this mounting procedure results in large transition zones between the ranges of oval and circular cross-sections, thus exhibiting again the above mentioned problems.